1. Field of the Invention
The present invention generally relates to a method and apparatus for reducing pollutants in the exhaust gases produced by the combustion of fuels. More particularly, the invention relates to such a method and apparatus wherein the reduction in pollutants is achieved by introducing hydroxyl radicals xe2x80x9cOHxe2x80x9d and other free radical intermediaries and oxidizers such as O, H, HO2 and H2O2 into the precombustion or postcombustion gas stream of a combustion engine.
2. Background
As is well-known in the art, an internal combustion engine draws in ambient air which is mixed with fuel for combustion in a combustion chamber or cylinder and the resulting exhaust gases are expelled. Ignition of the air/fuel mixture in the cylinder is typically achieved by an ignition device, such as, for example, a spark plug or the like, or adiabatic compression to a temperature above the fuel""s ignition point.
In certain internal combustion engines, such as for example, gasoline engines commonly in use today, air is inducted via an air intake duct or port which conveys the ambient air to a carburetor or a fuel injection arrangement where the air is mixed with fuel to create an air/fuel mixture. The air/fuel mixture is then conveyed via an intake manifold to the combustion chamber or cylinder of the engine. In diesel-type engines and engines using fuel-injection arrangements, the air and fuel are conveyed separately to the combustion chamber or cylinder of the engine where they are mixed.
After the air/fuel mixture has been burnt, the resulting exhaust gases are expelled from the combustion chamber to an exhaust manifold. The exhaust gases then may be conveyed by an exhaust pipe to the catalytic converter where pollutants are removed.
The flow of air to the combustion chamber, including the flow of the air/fuel mixture if applicable, as used herein is referred to as the precombustion gas stream, and the resulting flow of exhaust therefrom is hereinafter referred to as the postcombustion or exhaust gas stream. As used herein, the precombustion and postcombustion gas streams are collectively referred to as the combustion gas stream.
Internal combustion engines which operate by the controlled combustion of fuels produce exhaust gases containing complete combustion products of carbon dioxide (CO2) and water (H2O) and also pollutants from incomplete combustion such as carbon monoxide (CO), which is a direct poison to human life, as well as unburnt hydrocarbons (HC). Further, due to the very high temperatures produced by the burning of the hydrocarbon fuels followed by rapid cooling, thermal fixation of nitrogen in the air results in the detrimental formation of Nitrogen Oxides (NOx), an additional pollutant.
The quantity of pollutants varies with many operating conditions of the engine but is influenced predominantly by the air-to-fuel ratio in the combustion cylinder such that conditions conducive to reducing carbon monoxide and unburnt hydrocarbons (a fuel mixture just lean of stoichiometric and high combustion temperatures) cause an increased formation of NOx, and conditions conducive to reducing the formation of NOx (fuel rich or fuel lean mixtures and low combustion temperatures) cause an increase in carbon monoxide and unburnt hydrocarbons in the exhaust gases of the engine. Because in modern day catalytic converters NOx reduction is most effective in the absence of oxygen, while the abatement of CO and HC requires oxygen, preventing the production of these emissions requires that the engine be operated close to the stoichiometric air-to-fuel ratio because under these conditions the use of three-way catalysts (TWC) are possible, i.e., all three pollutants can be reduced simultaneously. Nevertheless, during operation of the internal combustion engine, an environmentally significant amount of CO, HC and NOx is emitted into the atmosphere.
Although the presence of pollutants in the exhaust gases of internal combustion engines has been recognized since 1901, the need to control internal combustion engine emissions in the United States came with the passage of the Clean Air Act in 1970. Engine manufacturers have explored a wide variety of technologies to meet the requirements of this Act. Catalysis has proven to be the most effective passive system.
Automotive manufacturers have generally employed catalytic converters to perform catalysis. The purpose is to oxidize CO and HC to CO2 and H2O and reduce NO/NO2 to N2. Auto emission catalytic converters are typically located at the underbody of the automobile and are situated in the exhaust gas stream of the engine, just before the muffler, which is an extremely hostile environment due to the extremes of temperature as well as the structural and vibrational loads encountered under driving conditions.
Nearly all auto emission catalytic converters are housed in honeycomb monolithic structures with excellent strength and crack-resistance under thermal shock. The honeycomb construction and the geometries chosen provide a relatively low pressure drop and a high geometric surface area which enhances the mass transfer controlled reactions. The honeycomb is set in a steel container and protected from vibration by a resilient matting.
An adherent washcoat, generally made of stabilized gamma alumina into which the catalytic components are incorporated, is deposited on the walls of the honeycomb. TWC technology for simultaneously converting all three pollutants comprises the use of precious or noble metals Pt and Rh, with Rh being most responsible for the reduction of NOx, although it also contributes to CO oxidation along with Pt. Recently less expensive Pd has been substituted for or used in combination with Pt and Rh. The active catalyst is generally about 0.1 to 0.15% precious or noble metals, primarily platinum (Pt), palladium (Pd) or rhodium (Rh).
Because the exhaust gases of the combustion engine oscillate from slightly rich to slightly lean, an oxygen storage medium is added to the washcoat which adsorbs (stores) oxygen during any lean portion of the cycle and releases it to react with excess CO and HC during any rich portion. Cerium Oxide (CeO2) is most frequently used for this purpose due to its desirable reduction-oxidation response.
The recent passage of the 1990 amendment to the Clean Air Act requires further significant reductions in the amount of pollutants being released into the atmosphere by internal combustion engines. In order to comply with these requirements, restrictions on the use of automobiles and trucks have been proposed, such as, employer-compelled car pooling, HOV lanes, increased use of mass transit as well as rail lines and similar actions limiting automobile and truck usage at considerable cost and inconvenience.
An alternative to diminished automobile and truck usage is decreasing emissions by increasing the efficiency of the internal combustion engine. This approach will have limited impact since studies show that most of automobile-originated pollution is contributed by only a small fraction of the vehicles on the road, these vehicles typically being older models having relatively inefficient engines and aging catalytic converters which inherently produce a lot of pollution. Any technological improvements to the total combustion process will not be implemented on these older vehicles if they require extensive or expensive modification to the engine or vehicle.
In addition, while considerable gains have been made in recent years to reduce the amount of pollutants in the exhaust gases of the internal combustion engine of vehicles such as automobiles and trucks, it is a considerable technological challenge and expensive to further reduce the amount of pollutants in the exhaust gases of the internal combustion, even though exhaust emissions of automobiles and trucks currently being manufactured do not meet proposed Environmental Protection Agency standards.
In lieu of decreasing exhaust emissions by increasing the efficiency of the internal combustion engine or decreasing the use of automobiles, a further alternative would be to increase the efficiency of the catalytic converter or catalysis. The conversion efficiency of a catalytic converter is measured by the ratio of the rate of mass removal of the particular constituent of interest to the mass flow rate of that constituent into the catalytic converter. The conversion efficiency of a catalytic converter is a function of many parameters including aging, temperature, stoichiometry, the presence of any catalyst poisons (such as lead, sulfur, carbon and phosphorous), the type of catalyst and the amount of time the exhaust gases reside in the catalytic converter.
Attempts to increase the efficiency of catalytic converters has not been sufficiently successful. Modern TWC catalytic converters help, but they are expensive, may have difficulty in meeting the future emission requirements, and have limitations in their performance lifetime. Catalytic converters also suffer from the disadvantage that their conversion efficiency is low until the system reaches operating temperature.
One object of the present invention is to provide a method and apparatus for reducing pollutants in the exhaust gases of an internal combustion engine without the need for major modifications to the internal combustion engine or the catalytic converter.
Another object of the invention is to provide a method and apparatus, which are inexpensive to employ and manufacture, and simple in structure and operation, for reducing pollutants of incomplete combustion in the exhaust gases of a combustion engine.
In accordance with the invention, it is believed that hydroxyl ion xe2x80x9cOHxe2x80x9d and other free radicals and oxidizers such as O, H, HO and H2O2 can be introduced into the combustion gas stream of a combustion engine to reduce pollutants and contaminants such as CO and HC. It has been observed that OH in the presence of oxygen can react rapidly with CO to produce CO2. It has also been observed that OH in the presence of oxygen can react rapidly with hydrocarbons (HC) to produce formaldehyde or other similar intermediary products which then further react with OH to form H2O, CO2, and OH. Moreover, there is evidence that the series of reactions does not consume, but rather regenerates OH.
In the case of CO, the following reaction steps convert CO to CO2 and regenerate OH:
CO+OHxe2x86x92CO2+H 
H+O2xe2x86x92HO2 
HO2+hxcexdxe2x86x92OH+O 
The latter process of dissociation of hydroperoxyl to hydroxyl can take place either via the absorption of ultraviolet (xe2x80x9cUVxe2x80x9d) photon or by thermal decomposition.
In the case of HC, a typical reaction set may involve the following steps:
HC+OHxe2x86x92HCHO 
HCHO+OHxe2x86x92H2O+HCO 
HCO+O2xe2x86x92CO2+HO 
Depending upon the HC species, there may be branching reactions and other free radical intermediaries and oxidizers such as O, H, HO2 and H2O2 may be produced and either enter into the reactions directly or through the products of other reactions such as:
O+O2xe2x86x92O3, or 
H2O2+hxcexdxe2x86x922OH 
Particularly important in the present invention is that OH is believed to be regenerated in the course of the reactions, i.e., it acts as a catalyst, and that the reaction sequence proceeds rapidly due to the strong nature of the free radical reactions.
It is believed that the presence of OH, and other free radical intermediates and oxidizers such as O, H, H2O2 and HO2, in the exhaust gases of a combustion engine leads, in the presence of requisite oxygen, to a very effective catalytic destruction of CO and hydrocarbons to non-polluting gas species CO2 and water vapor. The OH and other related free radicals and oxidizers created in the reactions can act as a catalyst independent of or in conjunction with the normal catalytic function of the precious metal particles (Pt, Pd, Rh and combinations thereof) in the catalytic converter.
It is believed that the injection of OH into the combustion gas stream results in rapid catalyzing of CO and HC reactions in the exhaust gas stream. The reactivity of OH is believed to cause much of the catalytic activity associated with the conversion of CO to CO2 and hydrocarbon to CO2 and H2O to take place in the gas phase and on the large surface area of the washcoat surface of the catalytic converter. Thus, within a small region near the entrance of the catalytic converter, the bulk of the reactions converting CO and HC to CO2 and H2O occurs. Because CO and the HC are oxidized in the gas phase and in the washcoat of the catalytic converter, with resulting substantial completion of the oxidation of CO and HC near the entrance to the catalytic converter, the bulk of the precious metal catalytic surface is freed from participating in these competing reactions. For example, the converter""s precious metal sites no longer need to catalyze the less reactive hydrocarbon species such as methane, ethane, ethene, benzene and formaldehyde. As a result, more effective catalytic activity at the precious metal sites can be directed toward reduction of nitrogen oxides to nitrogen and other non-polluting gas species.
It is believed that the action of the hydroxyl can take place over the volume of the exhaust gas and the entire surface area of the catalytic converter, i.e., over the entire, large area of the washcoat. This makes for a much larger, effective pollutant reduction action over the catalytic converter operating in the conventional manner. Under this new mode of catalytic conversion operation, nitrogen oxide reduction can diminish below conventional baselines. Alternatively, less precious metal content, or the use of less costly metals or their oxides can be used to reduce the nitrogen oxide compounds below allowable emission limits.
Several different modes of operation and devices may be utilized to carry out the invention. In one embodiment, OH is produced in a generator using mercury (Hg) vapor lamp radiation and atmospheric air intake which is conditioned to be of sufficiently high water vapor content, and preferably to about 100% saturation. It is believed that in air of high water vapor content there are two alternative competing reaction branches for creating OH. In the first case, there is direct photodissociation of the water into OH and H by the absorption of 185 nanometer (xe2x80x9cnmxe2x80x9d) photons. To achieve such high humidity, the water vapor can come from a heated water source or it can be supplied from the exhaust gas stream of the engine. The other reaction, which is favored at a lower, but still sufficiently high, water vapor content, is that the 185 nm ultraviolet (xe2x80x9cUVxe2x80x9d) radiation from the lamp acts on the air to produce atomic oxygen (O) and ozone (O3). The ozone is created by a three-body reaction involving atomic oxygen, molecular oxygen and any other molecular constituent of air, such as, for example, Nitrogen (N2), Oxygen (O2), Water (H2O) or Argon. The 253.7 nm UV radiation breaks down the ozone by photodissociation into molecular oxygen O2 and a metastable oxygen atom (O). If the air stream entering the generator has sufficient water vapor content, then it is believed the metastable atomic oxygen (O) combines with water molecules to form hydrogen peroxide:
O+H2Oxe2x86x92H2O2 
Further, the 253.7 nm UV radiation photodissociates the hydrogen peroxide into two hydroxyl molecules.
The generator thus injects ozone, atomic oxygen, hydrogen peroxide, and hydroxyl into the engine via for example, the intake manifold. It is believed that any hydrogen peroxide so injected will dissociate into hydroxyl under the high engine temperature. The hydroxyl which resides in the crevice regions of the combustion chamber should survive the combustion process in the engine and act upon the CO and HC remaining in the exhaust stream to produce CO2 and H2O according to the reactions described above.
A further embodiment of hydroxyl generation is to feed a water vapor-rich input air stream into a glow discharge generator (a generator in which a glow discharge occurs in water vapor primarily or only). Another approach is an overvoltage electrolysis cell to generate ozone in addition to oxygen and water vapor, followed by 200-300 nm UV exposure to create atomic oxygen by photodecomposition which in the presence of a water vapor-rich input air stream initiates hydrogen peroxide creation, followed by hydroxyl generation via UV dissociation of the hydrogen peroxide. This latter device can be very compact using a mercury vapor lamp as the source due to the high efficiency of the output at 253.7 nm and the high absorbability of ozone and hydrogen peroxide for UV light of this wavelength.
The foregoing embodiments principally involve generators injecting their streams of output gases into the intake manifold region of the engines. A natural advantage of such methods is that the low pressure condition in regions of the intake manifold provides a natural pumping mechanism. However, a drawback of these methods is that most of the highly chemically active species, including the free radicals such as hydroxyl, are destroyed in the combustion process and only those active species in the crevice regions and at the walls of the combustion chamber can effectively survive and enter into the exhaust gas stream where they are useful in oxidizing CO and HC. In contrast, generators which inject hydroxyl ion directly into or which create hydroxyl in the exhaust (postcombustion) gas stream can more effectively deliver the active species into the exhaust stream where CO and HC need to be oxidized. Thus, less chemically active species source strength would be required for equivalent emission reduction. This should translate directly into proportionally lower electrical input demands for the hydroxyl generator.
However, because of the higher pressures in the exhaust system, pumping is required to accomplish direct injection of the generator output into the exhaust gas stream. The use of a venturi will assist this process. Alternatively, because of the high vapor pressure of water at temperatures above approximately 120xc2x0 C., using a water vapor discharge source in the hydroxyl generator can also provide effective injection. Such water vapor can be collected by condensation or equivalent means from the exhaust system.
An embodiment creating hydroxyl in the exhaust gas stream is the irradiation of the exhaust gas stream with UV radiation in the 120 to 185 nm wavelength range which in the presence of sufficient water vapor produces catalytically active OH by direct photodissociation. A still further embodiment is the use of UV radiation in the 120 to 185 nm wavelength in an external generator using atmospheric air intake and water vapor collected from the exhaust gas stream and injecting water vapor, OH and H into the exhaust gas stream prior to or in the catalytic converter.
The means described above for creation of these free radical species and oxidizers include ultraviolet light-based generators, glow discharge generators, and overvoltage electrolytic cells plus UV radiation. Generator inputs can include electricity, water, air, oxygen, water vapor, water vapor plus air and water vapor plus oxygen.
Modes of possible introduction of the above species into the engine system include into the precombustion gas stream, such as the intake manifold, into the exhaust gas stream such as the exhaust manifold, and into the catalytic converter. The generators can be external or internal to these areas. A particularly advantageous feature of the external generator is that it provides the flexibility of installing the generator at a convenient location in the engine compartment or elsewhere on the vehicle. Another advantageous feature of the external generator embodiment is that the hydroxyl could be introduced at almost any desirable point in the intake or exhaust gas streams of the engine. A further advantageous feature of this embodiment is that the flow rate of hydroxyl from the hydroxyl generator is independent of engine speed, i.e., flow of air to the combustion chamber or flow of exhaust gases from the combustion chamber. Thus, at low engine speeds, the mass flow rate of hydroxyl will not be affected by low air mass flow through the combustion chamber. For external sources, means of pumping of the generator gas products can include natural low pressure areas in the engine, introduction of ventri regions, external pumps, or natural generator pressurization as with higher temperatures and water vapor sources.
Thus, the invention employs hydroxyl and its associated reaction species, O, H, H2O2 and HO2 to provide a catalytic cycle with OH playing the central role in reducing the CO and HC outputs of engines to meet present and future Ultra Low Emissions Vehicle xe2x80x9cULEVxe2x80x9d and Low Emissions Vehicle xe2x80x9cLEVxe2x80x9d standards. Because the OH acts as a catalyst, relatively small amounts of OH need to be injected for orders of magnitude more CO and hydrocarbons to be reduced to CO2 and H2O in the presence of oxygen in the exhaust gas stream.
An advantageous feature of the invention is that reduced emissions are achieved by adding hydroxyl radicals and other free radical intermediaries and oxidizers such as O, H, HO2 and H2O2 to modify the composition of the exhaust gases without the need to store special chemical additives onboard.
Yet another advantageous feature of the invention is that it can be applied to a variety of different types of engines including gas turbine and internal combustion engines, including, but not limited to, automobiles, trucks, stationary power generators, motorboats, motorcycles, motorbikes, lawn mowers, chain saws or leaf blowers which may use a variety of different fuels such as gasoline, gasoline-based formulations, diesel fuel, alcohol, natural gas and any other fuel where it is desired to reduce CO or HC.
It is believed that a further advantageous feature of the present invention is that due to the introduction of gas-phase catalyst species, whose activities occur over the whole catalytic converter surface, and the inherent reactivity of these species, much earlier catalytic conversion of CO and unburned HC will occur after engine start. In other words, the effective light-off delay time after engine start will be reduced as compared to the use of a typical catalytic converter.
In the case of combustion and other residential, commercial and industrial systems which have exhaust gas streams which contain volatile organic compounds (VOCs), but contain minimal or no nitrogen oxides such as from some industrial processes, there would be no need for the typical catalytic converter and certainly no need for a precious metal catalytic converter. This invention would provide for very low cost catalytic converter systems. In those situations where only CO or HC and other VOC""s are required to be oxidized, it is contemplated that a typical catalytic converter would not be required. However, it is contemplated that adequate time and/or a large surface area similar to that provided by the honeycomb structure of the typical catalytic converter would be necessary to allow the CO, HC and VOC oxidation reactions to take place.
These and other objects, advantages and features of the invention are achieved, according to one embodiment, by an apparatus comprising: 1) a combustion gas stream of an engine, 2) a catalytic converter for treating the exhaust gases in the combustion gas stream to reduce further the amount of at least one pollutant from incomplete combustion of fuel and/or oxides of nitrogen, and 3) a device for adding OH and associated free radicals and oxidizers to the combustion gas stream upstream from or at the catalytic converter to reduce further the amount of at least one pollutant in exhaust gases treated by the catalytic converter.
In accordance with the invention, a method is provided for treating exhaust gases to reduce at least one pollutant from incomplete combustion of a fuel having a precombustion gas stream of at least ambient air to the combustion chamber and a postcombustion gas stream of exhaust gases from the combustion chamber, the method comprising the steps of: adding hydroxyl and associated free radicals and oxidizers to at least one of the precombustion and the postcombustion gas streams and providing sufficient surface area in the postcombustion gas stream to allow the hydroxyl to treat the exhaust gases produced from the combustion of the fuel to at least reduce one pollutant from combustion.